Precipitator voltage control



y 3, 1969 J. w. CRENNING. ETAL I 3,443,358

PRECIPITATOR VOLTAGE CONTROL Filed June 11, 1965 Sheet of 2 PRECIPITATOR ENERGIZlNG SYSTEM FIG] AC SUPPLY SAMPLING POWER l PULSE Pu E POWER FLY 'fi -H??? as SUPPLY:

0 ----SAMPL\NG CYLCLE TIME sPARK-ovER L] Q il l2.

VOLTAGF VOLTAGE n E 0 7 F I M SPARK-OVEU Li Q m u 3B VOLTAGE H VOLTAGE r o v (H '1 INVENTORS M i 3 M U JOHN w. DRENNING mum B.THOMAS v BY ,W Jam-hr i Mfdq,

V ATTORNEY-Y J. w; DRENNINGY ET AL 3,443,358

PRECIPITA'IOR VOLTAGE CONTROL May 13, 1969 Filed June 11. 1965 I M 1 I I PRECIPITATOR ENERGIZING SYSTEM FIG-2 I l l I SWITCHING I PULSE MM J Ac SUPPLY 44 T I w 32 I EQ R 5% 364MB} 37- I 40 ig SUPPLY JI SUPFLfi/(Yfifi TOFTRBI 0 SAMPLING CYCLE m5 mm FIG-4A SPARK-OVER v m VOLTAGE WW H VOLTAG O "-SAMPLING CYCLE (HUME SPARK-OVER Li l! lUTUfL l VOLTAGE i mvsw-roas JOHN w. DRENNING JOHN B. THOMAS wr. VOLTAGE BY W M 4M3,

ATTORNEYS United States Patent 3,443,358 PRECIPITATOR VOLTAGE CONTROL John W. Drenning, Baltimore, Md., and John B. Thomas, Princeton, N.J., assignors to Koppers Company, Inc., Pittsburgh, Pa., a corporation of Delaware Filed June 11, 1965, Ser. No. 463,125 Int; Cl. B03c 3/68 US. Cl. 55--2 29 Claims ABSTRACT OF THE DISCLOSURE A method and system for maintaining precipitator voltage at a predetermined operating value below sparkover potential by superimposing one, or a pair, of unidirectional or high frequency sampling waveforms of predetermined amplitude recurrently on the continuously energized precipitator electrodes and adjusting the DC. potential applied to the electrodes in response to precipitator sparking during the application of the sampling waveforms.

This invention relates to a method and system for automatically controlling the energizing apparatus for electrostatic precipitators. More particularly, the present invention relates to maintaining the precipitator near but below spark-over voltage, thereby operating the system at maximum power.

The present invention controls the average or working potential applied to a precipitator in dependency on a sampling potential momentarily applied at recurrent intervals to detect the immediate value of the sparking potential. The response of the precipitator to the higher voltage of the sampling pulse determines whether it is desirable to maintain or correct the working potential.

It has been found advantageous in some circumstances to apply a series of substantially rectangular unidirectional potential impulses of relatively short duration to the precipitator electrodesprather than the conventional method of stepping-up alternating current and rectifying it to provide direct current for the electrodes. This particular type of energizing apparatus is utilized in the present invention and uses a pulse generator to supply power pulses to the electrostatic precipitator. It is desired to operate the pulse energization system in such a way so as to utilize maximum power but not to cause a sparkover or flash-over between the electrodes of the precipitator as such spark-over decreases the collection efficiency of the precipitator. It has been found that this objective is obtained by adjusting the magnitude of the power pulses to slightly less than the spark-over voltage.

However, the problem of detecting and maintaining optimum operating conditions is complicated by the fact that the optimum operating voltage is constantly fluctuating due to changes in the chemical and electrical properties of gases passing through the precipitator, the dust concentration on the electrodes, and changes in atmospheric conditions. Thus the pulses applied to the precipitator electrodes must be constantly adjusted in amplitude to stay within the maximum efficiency level of voltage. Heretofore, this has been accomplished by manual regulation of the input voltage, by spark rate control devices which provide predetermined sparking rates, or by control devices which gradually raise the input voltage and then quickly drop the voltage a preset amount when sparkover occurs. These systems are subject to inaccuracies and do not provide immediate control of a rapidly-changing spark-over voltage level.

A primary object of the present invention, therefore, is to provide automatic control over the energization voltage applied to a precipitator to provide optimum operating efficiency, particularly in a pulsed precipitator.

It is also an object of the invention to provide automatic control to a precipitator so that it consistently operates at slightly under its current spark-over voltage.

A further object is to provide a novel method of sampling the sparkover-over voltage at predetermined intervals in order to determine fluctuations in the spark-over level.

A further object is to provide a method of automatic precipitator control in which sampling pulses of different amplitudes are used to prevent excessive power dissipation in power pulse sparks.

These objects and advantages of the present invention are obtained by providing an automatic control device including a sequential power pulse and sampling pulse generator, sensing means to determine sparkover during a sampling pulse, and means responsive to the sensing means to vary the output of the pulse generator in order to follow a fluctuating spark-over voltage level.

The above objects and novel features of the invention will be more fully set forth in the following drawings and detailed description. It will be apparent that the drawings are for the purpose of illustration only and that various changes can be made in the type and arrangement of the elements disclosed without departing from the spirit or scope of the invention.

FIG. 1 is a schematic circuit diagram of an automatic precipitator control embodying the principles of the present invention.

FIG. 2 is a schematic circuit diagram of the embodiment of the present invention supplying two successive sampling pulses of different amplitude.

FIGS. 3A-C are graphs which display the operation of the system of FIG. 1.

FIGS. 4A-B are graphs which show the operation of the system of FIG. 2.

Referring to FIG. 1, the non-sampling pulses, or power pulses, are generated by the circuitry comprising a DC. power supply 32 with an output line, feeding current limiting resistor 1, vacuum switching tube 4, energy storage capacitor 7, charging diode 13, and blocking diode 12. Alternatively, an inductance may be used to replace resistor 1. Tube 4 is normally in a non-conducting state due to the negative bias voltage supplied to the control grid by direct current supply 17, which is shown schematically as a battery. Because tube 4 is normally nonconducting, capacitor 7 is therefore normally charged to a voltage Ea through resistor 1 and charging diode 13. Capacitor 7 supplies the working power supply voltage to the precipitator. Similarly, the sampling pulse circuitry consists of conventional DC. power supply 33 with an output line feeding resistor 2, vacuum switching tube 5 held normally nonconductive by battery 18, storage capacitor 8, charging diode 14, and blocking diode 11. Sampling pulse storage capacitor 8 is therefore normally charged to a voltage, Eb, which is the higher sampling voltage for the precipitator.

When a positive switching pulse from the sequential switching pulse generator unit occurs at the grid of switching tube 4 through capacitor 20, the tube conducts current. This conduction effectively begins to discharge capacitor 7 through blocking diode 12, causing a negative power pulse to occur on precipitator 16. Thus a potential difference is established between the electrodes of the precipitator 16 and ionization takes place. When the incoming switching pulse ceases, tube 4 stops conducting, capacitor 7 ceases discharging, and the energy fed to the precipitator is terminated. The voltage decay on the precipitator structure itself is not shown in the drawings and occurs at a rate determined by the immediate values of the discharge current and the capacity of the physical structure. The voltage before the onset of the next pulse depends on these parameters and the pulse interval. In the operation of the present invention, a series of switching pulses are sent from the switching pulse generator, thus energizing the precipitator by a series of short negative power pulses as illustrated in FIG. 3A by pulses 1,2,3 Nl.

When capacitor 7 stops discharging, it is recharged by power supply 32 through resistor 1 and diode 13 to ground. In actual practice, the interval between the power pulses is considerably longer than the duration of the pulse, so that energy replacement to the capacitor takes place over a relatively long period of time. The power pulses may be supplied with a duration from 0.01 to to 0.001 second with a duty ratio of 0.1 to 0.5.

At a predetermined time, in a manner to be subsequently described, one positive switching pulse is applied to tube through capacitor 21 in the interval following a power switching pulse and terminating before the next power switching pulse. This causes the negative sampling pulse N in FIG. 3A to be generated between precipitator power pulses by capacitor 8 through diode 11 to precipitator 16 in a manner similar to that associated with the power pulse circuitry. By making the supply voltage Eb larger than Ea, the sampling pulse applied to the precipitator will be larger in voltage amplitude than the power pulses. Therefore, as seen in FIG. 3A, a series of power pulses 1, 2, 3 N-1 and one larger sampling pulse N are applied to the precipitator electrode 16 during the sampling cycle. In the pulse sequence of FIG. 3A, the generation of a power pulse has been inhibited during generation of the sampling pulse N, which occurs between power pulses N l and N +1. In normal operating conditions, the power pulses then have slightly smaller magnitudes than the spark-over voltage illustrated by the dashed line. However, sampling pulse N has a magnitude greater than the spark-over voltage, thereby causing spark-over during the duration of the sampling pulse which is detected by suitable means subsequently described. Detection of spark-over only during the sampling pulse then denotes proper operation and power is delivered to the precipitator at maximum efliciency during the power pulses. In practice the power loss during the sampling pulse will be small since it occupies only a small portion of the time during a given sampling cycle. The sampling pulse rate may be a tenth of the power pulse rate, and the sampling pulse length may be equal to that of the power pulses or shorter by as much as a magnitude.

It is advantageous to maintain the voltage difference between Ea and Eb at a constant value so that the resultant sampling pulse and power pulse will have the constant relationship. The magnitude of each of the pulses will be dictated by the requirements of the precipitator, but in a typical situation the power pulse could be about 35 kv. and the sampling pulse about 37 kv. Normally the sampling pulse magnitude will be preset at some operable value as 5 percent higher than the power pulses.

The voltage supply control unit in FIG. 1 consists of diodes 25 and 26 which sense voltage pulses generated by the energization of the precipitator across resistors 23 and 24. For instance, in normal operating conditions the precipitator will spark when a sampling pulse occurs and a large signal is generated across resistor 24. When a power pulse occurs, the precipitator will not spark and the signal generated across resistor 23 is relatively small. The two signals are added through sensing diodes 25 and 26 and filtered with capacitor 27 and resistor 28 to produce a smooth signal of amplitude E The bias voltage E of battery 30 is chosen to equal the voltage level of the signal E during normal operation, or when the sampling pulse produces a spark and the power pulse does not. Thus during normal operation E E =0. This resultant is applied to a conventional servo-amplifier 31 whose output is zero when E E =O. Without an output from amplifier 31, servo-motor 35 does not rotate, variable auto-transformers 36 and 37 are not adjusted, and the outputs Ea and Eb of power supplies 32 and 33 remain unchanged. This insures normal operation of the system as shown in FIG. 3A for the next sampling cycle.

However, if the precipitator voltage is too high, as shown in FIG. 3B, many or all the power pulses as well as the sampling pulses will cause the precipitator to spark over. This will cause the signal E to be greater than E thus causing a positive resultant signal to amplifier 31. Motor 35 then rotates to decrease the outputs of power supplies 32 and 33 until only a sampling pulse sparks and the resultant signal again becomes zero. Conversely, if the spark-over voltage increases and neither the sampling pulse nor the power pulses cause a spark, as shown in FIG. 36, the amplifier 31 would receive a negative resultant signal and motor 35 would rotate to increase the outputs of power supplies 32 and 33.

The operation of the sequential switching pulse generator unit in FIG. 1 is controlled by an adjustable speed motor 38 which drives rotating alternating voltage generators 40 and 41. The speed of motor 38 may be adjusted by variable resistance 39. The motor and generators may be of miniaturized construction since their power requirements are negligible The alternating voltage from the generators triggers pulse generators 43 and 44 to produce a rectangular switching pulse for each voltage cycle from the alternating voltage generators. Thus the frequency of the power switching pulses from generator 43 and the sampling switching pulse from generator 44 is determined by the rotational speed of their respective alternating voltage generators. Mechanical gearing is provided so that the relative phase as well as frequency of each pulse generator is adjustable. The pulse width and therefore the duration of the power and sampling pulses may be individually set for optimum performance by adjustment of pulse generators 43 and 44. Thus, the sampling pulse is timed to occur between a pair of power pulses. Alternatively, sampling pulse switching generator 44 may when operated supply an inhibiting control voltage via lead 45 to generator 43. This connection permits the substitution of the sampling pulses for power pulses in the power pulse sequence, as shown in FIGS. 3 and 4.

From the foregoing explanation it may be seen the apparatus shown in FIG. 1 will maintain the amplitude of the power pulses applied to the precipitator at any desired level below spark-over voltage. However, as shown in FIG, 3B, when the power pulses are of too high voltage, considerable sparking will occur and unnecessary power dissipation results. This disadvantage may be overcome by the use of apparatus utilizing two sampling pulses, so that decreasing spark-over voltage may be detected before it reaches the level of the power pulses.

Such a system is illustrated in FIG. 2. In addition to the circuitry shown in FIG. 1, this embodiment of the invention contains means to generate and control a second sampling pulse. For the purpose of simplification, like numbering has been maintained in the two drawings for the common circuitry. As heretofore explained, when a switching pulse is not being applied storage capacitors 7 and 8 will be charged with a voltage equal to the output of their respective power supplies. In a similar manner, capacitor 9 will be charged to a voltage of Be through resistor 3 and charging diode 15.

The switching pulse generator unit now supplies two sampling switching pulses at predetermined intervals among the series of power switching pulses so that switching tubes 4, 5 and 6 are sequentially caused to conduct. As in the specific embodying of FIG, 1, operation of sampling pulse generators 44 and 46 preferably inhibit operation of generator 43 by the application of suitable gating or control voltages. The charging capacitors are periodically discharged through diodes 10, 11, and 12 into precipitator 16, creating a resultant energizing waveform series shown graphically in FIG. 4A. Power pulses 1, 2, 3 N+1, N+2 again constitute most of the sampling cycle, with first sampling pulse N and second sampling pulse M being spaced apart in a predetermined manner according to the adjustment of the switching pulse generator. The second sampling pulse M has a greater magnitude than the first sampling pulse N, which in turn has a greater magnitude than the power pulses, Typically, with the power pulse adjusted to 35 kv. and the first sampling pulse to 37 kv., the second sampling pulse M would be adjusted to about 38 kv.

In FIG. 2, sensing diodes 25 and 26 are now placed between the cathodes of sampling switching tubes 5 and 6. Thus in normal operation as illustrated in FIG. 4A, first sampling pulse N would not cause the precipitator to spark-over, "but second sampling pulse M would cause a spark to occur. Diode 25 would then sense a small voltage across resistor 23 and diode 26 would sense a much larger spark-over pulse across resistor 24. The added voltages are applied to the filter comprised of capacitor 27 and resistor 28 and the smoothed output signal E is then subtracted from bias voltage E E is again chosen so that E E =0 during normal operation. If the resultant signal is zero, the servo-amplifier 31 will not produce an output, servo-motor 35 will not run, and variable autotransformers 50, 36, and 37 are not varied to change the outputs of the power supplies.

However, if the spark-over voltage decreases and both of the sampling pulses cause a spark-over, the increased signal on resistor 23 will cause E to exceed E and a positive signal is applied to the servo-system. Motor 35 will rotate to decrease the outputs of power supplies 32, 33, 34, thereby decreasing the magnitude of the power and sampling powers. However, only limited spark-over and subsequent power loss may be seen to have resulted as no continuous spark-over occurred from the power pulse series. Conversely, if neither sampling produces a spark, the amplifier 31 receives a negative resultant signal and servo-motor 35 rotates to increase Ea, Eb, and Be. Referring to FIG. 4B, the duration of the sampling pulse may be varied by adjustment of pulse generators 44 and 45. In this embodiment, the pulse lengths of the sampling pulses are made considerably shorter than the lengths of the power pulses. Thus, although deliberate spark-over is caused by the sampling pulses, little power loss results because of their short duration.

The control system of the invention provides means whereby a complete precipitator system may be automatically controlled in a manner to give consistent maximum operating efficiency.

What is claimed is: 1. A system for controlling the power supplied to an electrostatic precipitator comprising:

adjustable output voltage working power supply means electrically connected to the precipitator electrodes,

means electrically connected to the precipitator electrodes for recurrently applying a momentary higher sampling voltage to the precipitator electrodes, and

voltage control means for said power supply means operative in dependency on precipitator response during the application of said higher sampling voltage.

2. The system of claim 1 further comprising:

means for raising the voltage of the working power supply means on absence of spark over current surge during higher voltage application.

3. The system of claim 1 further comprising:

means for lowering the voltage of the working power supply means on the presence of spark over current surge during higher voltage application.

4. The system of claim 1 further comprising:

means for lowering the voltage of the working power supply means on spark over current surge at working power supply output voltage.

5. The system of claim 1 further comprising:

means disconnecting the working power supply from the precipitator during operation of the second recited means.

6. An electrostatic precipitator comprising:

adjustable output voltage working power supply means,

pulse means for recurrently applying a first momentary sampling voltage to the precipitator electrodes at a voltage higher than the working voltage and a second momentary sampling voltage at a voltage higher than the first sampling voltage, and

precipitator current responsive control means for the power supply means operative on spark over current surge during the first sampling voltage to reduce power supply voltage and operative on absence of spark over current surge during the second sampling voltage to raise power supply voltage.

7. An electrostatic precipitator comprising:

a pair of electrodes,

power pulse supply means for supplying a series of power pulses for energizing said pair of electrodes near its spark over voltage, and

at least one higher voltage sampling pulse supply means for recurrently sampling the spark over voltage of said pair of electrodes.

8. The system of claim 7 further comprising:

means responsive to current levels in said precipitator means to vary the output voltages of said pulse supply means.

9. The system of claim 7 wherein:

the sampling pulse supply means is operative to supply pulses of substantially shorter duration than the power pulse supply means.

10. A system for energizing an electrostatic precipitator comprising:

a plurality of power supplies each having an output of a different voltage magnitude,

energy storing means separately connected to the outputs of said power supplies,

precipitator electrode means connected to for energization by the outputs of said energy storing means, and

sequential switching means connected to the outputs of said power supplies for causing said energy storing means recurrently to sequentially energize said precipitator means.

11. The system of claim 10 wherein said sequential switching means comprises:

a plurality of normally open switching means connected to the output of said energy storage means, and pulsing means connected to said switching means to sequentially close said switching means.

12. The system of claim 11 wherein said switching means comprises:

normally blocked vacuum switching tubes unblocked in response to said pulsing means.

13. An electrostatic precipitator comprising:

a plurality of variable voltage output power supplies,

a power pulse storing means connected to the output of one power supply for delivering a series of recurrent power pulses,

sampling pulse storing means connected to the re maining power supplies to deliver recurrent sampling pulses,

precipitator electrode means connected to the one pulse storing means for energization at a point near spark over voltage,

sequential switching control means for causing said pulse storing means to deliver a predetermined series of power and sampling pulses to said precipitator electrode means,

sensing means responsive to precipitator current to provide an output signal, and

power supply voltage varying means connected between said sensing means and said power supplies to vary the output of the power supplies as the precipitator performance changes.

14. The system of claim 13 wherein the sequential switching control means is operative to deliver sampling pulses of substantially shorter duration than the power pulses.

15. The system of claim 13 wherein the power supply varying means comprises:

servomechanism means which varies the output voltage of said power supplies in accordance with the magnitude of the signal received from said sensing means.

16. The system of claim 15 wherein said sensing means comprises:

adding means responsive to current delivered by a plurality of pulse storing means,

filtering means for smoothing the output of said adding means, and

subtracting means responsive to the output of said filtering means to provide a resultant control signal.

17. In an automatic voltage regulator for maintaining the voltage level of the electrodes of an electrostatic precipitator near spark over voltage:

a power supply which produces a series of power pulses to energize said electrodes,

first sampling pulse supply means for detecting the decrease of the spark over voltage of said electrodes, and

second sampling pulse supply means for detecting the increase of said spark over voltage.

18. The apparatus of claim 17 further comprising:

means to vary the outputs of said pulse supply means in accordance with variations of the spark over voltage detected by said sampling pulses.

19. The method of operating an electrostatic precipitator with an adjustable working voltage supply comprising:

energizing the precipitator electrodes under applied working voltage,

recurrently momentarily increasing the precipitator electrodes voltage above the applied working voltage, sensing precipitator response, and

raising the working voltage in absence of spark over current surge during the momentarily increased voltage.

20. The method of claim 19 further comprising:

reducing the working voltage on spark over current surge in the absence of the momentarily increased voltage.

21. The method of operating an electrostatic precipitator with an adjustable working voltage supply comprising:

energizing the precipitator electrodes under applied working voltage,

recurrently momentarily increasing the precipitator electrodes voltage to first and second difierent values above the applied working voltage,

sensing precipitator response,

raising the working voltage in absence of spark over current surge during the higher momentarily increased voltage, and

reducing the working voltage on spark over current surge during the lower momentarily increased voltage.

22. In the method of operating an electrostatic precipitator under an adjustable applied working voltage, the steps of:

recurrently momentarily applying increased voltage and sensing precipitator response to determine the relation of applied working voltage to present spark over voltage;

23. The method of controlling the voltage supply to an electrostatic precipitator comprising:

applying to the precipitator electrodes a series of power pulses with magnitudes slightly lower than the spark over voltage,

applying to the precipitator electrodes at predetermined times sampling pulses of a voltage magnitude slightly greater than the power pulses,

sensing the response of said precipitator when energized by a sampling pulse, and varying the voltage magnitude of the power pulses according to the sensed response of the precipitator. 24. The method of claim 23 further comprising: the step of increasing the voltage magnitude of the pulses if no spark over occurs during the sampling pulse. 25. The method of controlling the voltage supply to an electrostatic precipitator comprising:

applying to the precipitator electrodes a series of spaced equal voltage magnitude power pulses, applying to the precipitator electrodes at first predetermined times first sampling pulses of slightly greater voltage magnitude than the power pulses, applying to the precipitator electrodes at second predetermined times second sampling pulses with a magnitude greater than said first sampling pulse, sensing the response of said precipitator, and varying the voltage magnitude of the pulses according to the sensed response of the precipitator. 26. The method of claim 25 wherein: the magnitude of the pulses is increased if no spark over occurs during the second sampling pulses. 27. The method of claim 25 wherein: the voltage magnitude of the pulses is decreased if spark over occurs during the first sampling pulse. 28. The method of claim 25 wherein: the application of the sampling pulses is of shorter duration than the duration of the power pulses. 29. Means for controlling the proximity of the spark over voltage of an electrostatic precipitator with its operating voltage comprisingz voltage pulse generating means connectible with the electrodes of a precipitator to increase the voltage differential thereacross, current variation sensing means connectible with said precipitator to supply an output signal in response to spark over, servo-motor means connectible to a variable output voltage precipitator power supply, and control means for activating the servo-motor on absence of signal from the sensing means during pulse generation and for reversely activating the servomotor on signal from the sensing means during absence of pulse generation.

References Cited UNITED STATES PATENTS 1,865,907 7/1932 Heinrich 55123 1,934,923 11/1933 Heinrich 55l39 X 1,959,374 5/1934 Lissman 552 1,978,426 10/1934 Hahn 55l39 2,000,019 5/ 1935 Heinrich et al. 552 2,509,548 5/1950 White 55l39 2,623,608 12/1952 Hall 317-l57 X 2,642,149 6/ 1953 Backer et al. 55105 2,666,496 1/ 1954 Willison 55105 X 2,672,208 3/1954 Van Hoesen 55--139 2,675,092 4/1954 Hall 55-105 2,767,804 10/1956 Foley 55104 2,841,239 7/1958 Hall et al. 55105 2,925,142 2/1960 Wasserman 55105 2,935,155 5/1960 Foley 55105 2,978,065 4/1961 Berg 55105 2,992,699 7/ 1961 J arvinen 55105 3,039,252 6/1962 Guldemond et al 55105 3,039,253 6/1962 Van Hoesen et a1. 55105 J. L. DE CESARE, Primary Examiner.

U.S. Cl. X.R. 

